I wanted to try this idea out but after i done a bit of research and found out the actual price of the false rock sheets lets just say they are not cheap so i didn't go ahead with the project, never the less its a really good idea if you can afford it

I have tried this stuff before but turned away from it for a number of reasons. It was too pricey, too brittle and not so good at taking paint – the surfaces were too smooth, so the paint streaked. By the time I'd got it the way I wanted it, I figured I might as well have worked with polystyrene, PVA and sand.

I have a buddy that works in theatrical set design. I've seen him do some crazy stuff with styrofoam. One of the techniques the use to get a rough rock look is to sprinkle a chemical that eats the foam on it. He uses a brush and flings the stuff at the foam. That gives you rough streaks. It might be hard to get a look that is suitable for miniature work. theatrical work can be a lot more rough because no one sees it up close.

You might be able to apply it with a brush in light strokes to eat away small portions of the foam in a more controlled way. You could also lightly sand it and then apply a PVA/sand mixture to it.

I'm not sure what chemicals he uses but I think it's just paint thinner.